Device produced by a process of controlling grain growth in metal films

ABSTRACT

A process for controlling grain growth in the microstructure of thin metal films (e.g., copper or gold) deposited onto a substrate. In one embodiment, the metal film is deposited onto the substrate to form a film having a fine-grained microstructure. The film is heated in a temperature range of 70-100° C. for at least five minutes, wherein the fine-grained microstructure is converted into a stable large-grained microstructure. In another embodiment, the plated film is stored, after the step of depositing, at a temperature not greater than −20° C., wherein the fine-grained microstructure is stabilized without grain growth for the entire storage period.

TECHNICAL FIELD

[0001] The present invention relates, in general, to the process ofmaking thin film depositions on a substrate and, more specifically, to aprocess for controlling the grain structure growth of a thin metal filmafter having been deposited on a substrate.

BACKGROUND OF THE INVENTION

[0002] A large variety of thin films are used in the fabrication of verylarge scale integrated circuit devices. These films may be thermallygrown or deposited on a substrate. The thin films may be metals,semiconductors, or insulators.

[0003] There are several techniques for depositing films on a substrate.One such technique may be performed in a vacuum chamber and is known asphysical vapor deposition or sputtering. Another technique may beperformed in a bath and is known as electroplating.

[0004] It is known that sputter deposited copper films have acharacteristic as-deposited microstructure which changes as a functionof time at room temperature. This phenomenon has been documented by J.W. Patten et al. in “Room Temperature Recrystallization in Thick BiasSputtered Copper Deposits,” Journal of Applied Physics, vol. 42, No. 11,pages 4371-77 (Oct. 1971).

[0005] Work has shown that, for electroplated copper, the as-platedcopper has a fine-grained microstructure, with an average crystallitesize of less than 100 nanometers. Verification of this microstructure inthe as-plated film has been established using Back-Scattered KikuchiDiffraction (BKD). When stored at room temperature, no change has beenobserved in the fine-grained microstructure for a period of 8-10 hours;this time period is known as the incubation period. After the incubationperiod, grain growth has been observed for the next 10-20 hours, withthe microstructure then reaching an apparent steady-state having anequilibrium structure.

[0006] In order to take advantage of the fine-grained microstructure ofcopper, certain critical process steps must be carried out within 20hours after copper electroplating. This requirement is difficult to meetin a manufacturing environment in which, for example, the as-depositedsubstrate may sit on a shelf over the weekend.

[0007] It is also known that grain growth in the microstructure ofcopper may be accomplished within the space of several minutes byheating the copper at a high temperature. Heating a metal to change itsmicrostructure is an established metallurgical practice. It has beenbelieved, however, that for a metal such as bulk copper, a relativelyhigh temperature of at least 350° C. is required to obtain anyappreciable change in its microstructure. This has been documented in“Metals Handbook,” Vol. 4, pages 719-28 (9th ed., American Society forMetals, Metals Park, Ohio, 1981).

[0008] Several semiconductor manufacturers are replacing the currentaluminum interconnect metallization with copper wiring, because copperoffers superior electrical conductivity and electromigrationperformance. Several deposition techniques are possible, one of thesebeing electroplating. Electroplating has advantages: it has excellenttrench fill properties and produces a copper film with near zeroresidual stress.

[0009] Electroplated copper interconnects may be used in multi-chipmodules for both power distribution and signal transmission. In the morecomplex structures, multiple levels of wiring may be required. Thefabrication of these wiring levels is well known in the art. The grainstructure of the plated copper is critical. If the plated copper has afine-grained microstructure, the etching results in a smooth surface. Ifthe plated copper has a large-grained microstructure, however, theetching results in a rough surface. The rough surface has a disadvantagebecause such a surface precludes an accurate measure of the thickness ofthe polyimide layer which is deposited over the wiring.

[0010] The deficiencies of the conventional processes used to depositthin metal films on a substrate show that a need still exists for aprocess which can control the grain structure growth of a thin metalfilm after the film has been deposited on a substrate.

SUMMARY OF THE INVENTION

[0011] To meet this and other needs, and in view of its purposes, thepresent invention provides a process of depositing thin metal filmshaving different microstructures onto a substrate. The process controlsgrain growth in the microstructures and, in one embodiment, includes thefollowing steps:

[0012] (a) a metal film is deposited onto the substrate to form a filmhaving a fine-grained microstructure, and

[0013] (b) the metal film is heated in a temperature range of 70-100°C., for at least five minutes, to convert the fine-grainedmicrostructure into a stable, large-grained microstructure.

[0014] In another embodiment, the metal film is frozen, after the step(a) of depositing, at a temperature not greater than −20° C., whereinthe fine-grained microstructure is stabilized without grain growth forthe entire freezing period.

[0015] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary, butare not restrictive, of the invention.

BRIEF DESCRIPTION OF THE DRAWING

[0016] The invention is best understood from the following detaileddescription when read in connection with the accompanying drawing.Included in the drawing are the following figures:

[0017]FIG. 1 is an x-ray diffraction scan of an electroplated copperfilm with a fine-grained microstructure;

[0018]FIG. 2 is an x-ray diffraction scan of an electroplated copperfilm with a large-grained microstructure; and

[0019]FIG. 3 is a schematic plot of sheet resistance versus time for anelectroplated copper film removed from the plating bath.

DETAILED DESCRIPTION OF THE INVENTION

[0020] Electroplated copper films have a characteristic microstructurewhen immediately removed from a plating bath. This microstructure ishereinafter referred to as a “Type A” microstructure. The Type Amicrostructure is fine-grained, with an average crystallite size of lessthan 100 nanometers. Verification of the fine-grained structure in theas-plated film has been established using Back-Scattered KikuchiDiffraction (BKD).

[0021] No change has been observed in the structure for a period of 8-10hours; this time may be termed the incubation period. After theincubation period, grain growth has been observed for the next 10-20hours, with the microstructure then reaching an apparent equilibriumstructure, or steady state. This new microstructure is hereinafterreferred to as a Type B microstructure. The Type B microstructure haslarge grains, with an average crystallite size greater than 1,000nanometers.

[0022] If Type A copper is allowed to remain at room temperature(approximately 25° C.) for a period of at least 24 hours, it will beconverted to Type B copper. Type B copper can be held at roomtemperature for extended periods of time (more than 30 days) withoutchange in the microstructure.

[0023] X-ray diffraction scans of electroplated copper films with Type Aand Type B microstructures are shown in FIG. 1 and FIG. 2, respectively.The scan in FIG. 1 was taken 30 minutes after removal of the copper filmfrom the plating bath. The scan in FIG. 2 was taken approximately 45hours after removal from the plating bath. The copper film was held atroom temperature for the entire duration of the experiment.

[0024] As shown in the figures, a Type A microstructure may becharacterized by a large coherent diffracting domain which produces avery broad peak in an x-ray diffraction pattern. A Type B microstructureproduces narrow peaks in an x-ray diffraction pattern relative to a TypeA microstructure. A good reference for x-ray diffraction techniques usedto characterize polycrystalline materials, such as copper, is B. D.Cullity, “Elements of X-Ray Diffraction,” pages 281-323 (2d ed.,Addison-Wesley, Reading, Mass., 1978).

[0025] The initiation of the transformation from Type A to Type B coppermay be delayed for an extended period of time (at least 92 hours) byplacing the copper in a controlled temperature environment at atemperature of −20°C. or lower. Moreover, the conversion from Type A toType B copper may be accomplished within the space of several minutes byheating the copper at a low temperature (greater than 60° C. but lessthan 100° C.).

[0026] This rapid conversion of plated copper film from Type A into TypeB is unexpected. As mentioned earlier, it has been thought that a metalsuch as bulk copper must be heated to at least 350° C. in order toobtain any appreciable change in its microstructure.

[0027] The inventors measured the rate of transformation of Type A toType B copper using in-situ, high-temperature, x-ray diffractiontechniques. The table below lists the times to complete thetransformation at room temperature for plated copper which had beenheated previously at various temperatures from 25° C. to 60° C. TABLE 1Transformation of Copper From Type A to Type B Storage Temperature inTime to Complete Degrees Centigrade Transformation in Hours 25 30 40 3.550 1.2 60 0.19

[0028] J. W. Patten et al. teach that the resistance of sputtered copperchanges as a function of time while the copper is held at roomtemperature. Thus, resistance may be used as an alternative to x-raydiffraction as a monitor of the copper transformation. FIG. 3 is aschematic illustration of the variation in sheet resistance of a blanketcopper film. The plot consists of an initial high resistance state,which corresponds to the Type A microstructure. After a period of time,the resistance drops until it reaches a final steady state, whichcorresponds to the Type B microstructure. The time required to reach thefinal steady state as well as the time before the transformation occurs(termed the “incubation period” depend on factors such as the depositiontechnique, thickness of the copper film, and temperature at which thecopper film is held after deposition.

[0029] It is desirable, of course, to use plated copper with a lowresistance in the chip or chip carrier when making wires becauseresistance is critical to the performance of the copper wire. It mayalso be desirable to plate copper with a fixed sheet resistance in orderto obtain a specific resistance for the as-plated copper film.

[0030] It will be understood that the time to complete transformation ofthe as-deposited copper film from Type A to Type B may be shorteneddramatically (as shown in Table 1) and, consequently, so may be the timeto decrease the sheet resistance of the plated copper film.

[0031] A process has thus been found to produce a copper film with asteady and stable large-grained microstructure (Type B) which has alower sheet resistance than the original as-deposited copper film. Apreferred process includes rinsing the substrate immediately aftercopper plating. The rinsing step is done with hot deionized water havinga temperature of at least 70° C., but not more than 100° C., for a timenot less than five minutes. The preferred process relies on the factthat the grain growth has been shown by the inventors to be a purelykinetic process.

[0032] This process has been verified in experiments with good results.Wafers 100 mm in diameter with five microns of electroplated copper filmhave been processed using 70° C. deionized water rinse and shown to haveachieved a large-grained equilibrium state (Type B).

[0033] Various other approaches have also been taken, such as (1)furnace annealing after plating, and (2) plating other additives intothe substrate. Furnace annealing after plating has been successful. Thepreferred process is superior over furnace annealing, however, in thatit can readily be integrated into the plating assembly line.

[0034] The preferred process thus provides an approach by which theconversion of a Type A microstructure into a Type B microstructure maybe accelerated. This acceleration is accomplished by heating the platedsubstrate at a temperature below 100° C. The degree of acceleration willbe dependent on the temperature selected. For example, an electroplatedcopper film, placed in a tank of deionized water held at 70° C. for atleast five minutes, will convert from a Type A microstructure to a TypeB microstructure.

[0035] It will be understood that furnace annealing may also be aprocess in which the conversion of a Type A microstructure into a Type Bmicrostructure may be accelerated. Again, this acceleration may beaccomplished by heating the as-plated substrate in an oven at atemperature below 100° C. The degree of acceleration will depend on thetemperature selected. For example, an electroplated copper film, placedin an oven held at 70° C. for at least five minutes, will convert from aType A microstructure to a Type B microstructure.

[0036] Furthermore, because the grain growth in a plated metal appearsto be kinetically driven, a process has been found to slow or preventgrain growth. Stated differently, a process has been found to slow orprevent the initiation of the conversion of a Type A plated film. Thisprocess may be accomplished by maintaining the as-deposited metal filmat a temperature below that of room temperature. The extent of the delaywill depend on the temperature selected. For example, maintaining coppermetal at −20° C. will delay the initiation of the conversion for aperiod of at least 30 days.

[0037] The aforementioned process has been found useful in the processof fabricating electroplated copper interconnects or wires in asubstrate. As previously mentioned, the grain structure of the platedcopper is critical during the etching process. If the plated copper hasa fine-grained microstructure, the etching results in a smooth surface.If the plated copper has a large-grained microstructure, however, theetching results in a rough surface. The rough surface has disadvantages.

[0038] The inventors have found that the roughening of the copper may bedue to room temperature grain growth. The grain growth may result inlarge grains of copper with different crystallographic planes orientedto the surface. These planes may etch at different rates during theetching process, thereby resulting in rough copper. This rough coppermay also form etch pits. The etch pits may make for an undesirable metalinterface topography with any subsequent metal layers. The etch pits mayfurther tend to trap process waste and debris and thereby contaminatethe metal-to-via interface as well. This interface may open electricallyafter thermal cycling, resulting in low production yield and in-fieldreliability problems.

[0039] It has been observed that, if the etching is performedimmediately after plating, the resulting copper may have a smoothsurface texture. Such a surface will permit an accurate measure of thethickness of the polyimide over-layer and will form a good metallurgicalbond to a connecting via. From a manufacturing vantage, however, theetching cannot be performed immediately. A delay of 24 hours may be anabsolute minimum and a delay of 72 hours (over a weekend) may berequired for routine manufacturing conditions.

[0040] Therefore, in order to obtain uniform properties and behavior, itis desirable to stabilize the copper microstructure prior to subsequentprocess steps. The process by which this result may be accomplished isto refrigerate the substrate immediately after plating until themanufacturing line is ready to etch the seed layer.

[0041] The following experiment was performed. A 5-micron thick copperfilm was plated on a 100 mm Si wafer using the tool for substrate copperplating. The wafer was quartered immediately after plating. One quarterwas used as a control and held at room temperature for 24 hours, whilethe other three quarters were placed in a commercial freezer and held at−20° C. The three samples placed in the freezer were removed from thefreezer at 24-hour intervals. X-ray diffraction scans showed that thecontrol sample had a Type B microstructure while the three frozensamples had Type A microstructures. No grain growth was observed in thefrozen samples. Thus, a process has been found to prevent the initiationof conversion of a Type A microstructure before etching by maintainingthe as-deposited metal film at a temperature below −20° C. until themanufacturing or assembly line is ready to perform the etching process.

[0042] The present invention also provides yet another process in whichan intermediate microstructure, hereinafter referred to as a Type Cmicrostructure, may be obtained. It will be understood that a Type Cmicrostructure is one which has a grain size greater than that of a TypeA microstructure, but less than that of a Type B microstructure.

[0043] A Type C microstructure may be obtained in a two-step process.Step 1 of the process consists of heating the as-deposited metal film ata temperature below 100° C. while monitoring the change in itsmicrostructure by a temperature-independent technique, such as x-raydiffraction or sheet resistance measurements. Once the desiredmicrostructure is reached, Step 2 may be performed. Step 2 consists ofimmediately reducing the temperature of the partially transformed metalfilm to a value no greater than -20° C. The film may be kept in thefreezer until it is ready for the next step in the assembly line, suchas etching.

[0044] Although illustrated and described herein with reference tocertain specific embodiments, the present invention is nevertheless notintended to be limited to the details shown. Rather, variousmodifications may be made in the details within the scope and range ofequivalents of the claims and without departing from the spirit of theinvention. It will be understood, for example, that the presentinvention is not limited to only copper films. Rather, the invention maybe extended to other metals beyond copper. Room temperature grain growthhas been reported in gold films. This report suggests that one mayextend the invention to gold films and, possibly, to all Group IBmetals.

[0045] It will be further understood that the present invention may beextended to any deposition process for metal films. As such, thedeposition may be by way of electroplating techniques in a bath or bysputtering techniques in a vacuum.

What is claimed:
 1. A process for controlling grain growth in themicrostructure of thin metal films deposited onto a substrate, theprocess comprising the steps of: (a) depositing a metal film onto thesubstrate to form a film having a fine-grained microstructure, and (b)heating the metal film in a temperature range of 70-100° C. for at leastfive minutes, wherein the fine-grained microstructure is converted intoa stable large-grained microstructure.
 2. The process of claim 1 whereinstep (b) includes immersing the metal film in a de-ionized watersolution having a temperature range of 70-100° C.
 3. The process ofclaim 2 wherein the metal film is one of copper and gold.
 4. The processof claim 3 wherein the depositing step includes electroplating the metalfilm onto the substrate.
 5. The process of claim 1 wherein step (b)includes placing the metal film in an oven having a temperature range of70-100° C.
 6. The process of claim 5 wherein the metal film is one ofcopper and gold.
 7. The process of claim 6 wherein the depositing stepincludes electroplating the metal film onto the substrate.
 8. A processfor controlling grain growth in the microstructure of thin metal filmsdeposited onto a substrate, the process comprising the steps of: (a)depositing a metal film onto the substrate to form a film having afine-grained microstructure, and (b) storing the metal film at atemperature not greater than −20° C., wherein the fine-grainedmicrostructure is stabilized without grain growth for the entire storageperiod.
 9. The process of claim 8 wherein the metal film is one ofcopper and gold.
 10. The process of claim 9 wherein the depositing stepincludes electroplating the metal film onto the substrate.
 11. In theformation of multi-layered wiring on a substrate, a process forcontrolling grain growth in the wiring comprising the steps of: (a)depositing a seed layer onto the substrate, (b) depositing a metal filmonto the seed layer to form a film having a fine-grained microstructure,(c) storing the metal film at a temperature not greater than −20° C.,and (d) etching the metal film immediately after the step of storing,wherein the fine-grained microstructure is stabilized without graingrowth.
 12. The process of claim 11 wherein the plated film is one ofcopper and gold.
 13. The process of claim 12 wherein the seed layer is alayer of copper less than 200 nanometers thick.
 14. A process forcontrolling grain growth in the microstructure of thin metal filmsdeposited onto a substrate, the process comprising the steps of: (a)depositing a metal film onto the substrate to form a film having afine-grained microstructure, (b) heating the metal film while monitoringthe fine-grained microstructure, (c) stopping the step of heating whenthe fine-grained microstructure changes to a second microstructurehaving a second type of grain, and (d) freezing the secondmicrostructure at a temperature not greater than −20° C., wherein thesecond microstructure is stabilized for the entire freezing period.